# 6.3. Validate Return Value

Decorates methods and getter properties to validate their return values against a Joi schema.

## Import

```typescript
import { ValidateReturnValue, ValidateReturnValueError } from '@triplef/config-factory/validate-return-value';
```

## ValidateReturnValue Decorator

### Usage

```typescript
import Joi from 'joi';
import { ValidateReturnValue } from '@triplef/config-factory/validate-return-value';

const UserSchema = Joi.object({
  id: Joi.number().required(),
  name: Joi.string().required(),
});

class UserService {
  @ValidateReturnValue(UserSchema)
  getUser(id: number) {
    return { id, name: 'John' };
  }

  @ValidateReturnValue(UserSchema)
  get user() {
    return { id: 1, name: 'Jane' };
  }
}

const service = new UserService();
service.getUser(1); // Validates return value
service.user;      // Validates getter return value
```

### On Getters

```typescript
const ConfigSchema = Joi.object({
  port: Joi.number().port().required(),
  env: Joi.string().valid('development', 'production').required(),
});

class AppConfig {
  @ValidateReturnValue(ConfigSchema)
  get config() {
    return { port: 3000, env: 'development' };
  }
}
```

### On Methods

```typescript
const ApiResponseSchema = Joi.object({
  data: Joi.any().required(),
  timestamp: Joi.date().iso().required(),
});

class ApiService {
  @ValidateReturnValue(ApiResponseSchema)
  fetchData(endpoint: string) {
    return {
      data: { endpoint },
      timestamp: new Date(),
    };
  }
}
```

## ValidateReturnValueError

Thrown when a return value fails Joi validation.

### Class Definition

```typescript
class ValidateReturnValueError extends Error {
  constructor(message?: string, cause?: unknown) {
    super(message, { cause });
    this.name = 'ValidateReturnValueError';
  }
}
```

### Properties

| Property | Type | Description |
|----------|------|-------------|
| `name` | `string` | Always `"ValidateReturnValueError"` |
| `message` | `string` | Error message (e.g., `"Schema violation"`) |
| `cause` | `unknown` | Joi validation details array |

### Usage

```typescript
import { ValidateReturnValueError } from '@triplef/config-factory/validate-return-value';

try {
  const result = service.getUser(1);
} catch (error) {
  if (error instanceof ValidateReturnValueError) {
    console.log(error.cause); // Joi validation details
  }
}
```

## Note

This decorator is used internally by `@CacheReturnValue` when a Joi schema is provided. You can use it standalone for validation without caching.

## See Also

- [6.2. Cache Return Value](6.2-cache-return-value.md) - Decorator that combines caching with validation